bdb_equality_candidates index_param failed

Hi, I have been receiving quite a few of these entries in my logs, and
so far searches on the net have no provided me with any clues as to
what it means, or how I can fix it:
Apr 13 23:27:46 mars slapd[8630]: <= bdb_equality_candidates:
(uidNumber) index_param failed (18)
My relevant details:
Berkeley Sleepycat db 4.2.52
Openldap 2.1.25
slapd.conf:
loglevel 64
database bdb
cachesize 10000
suffix "dc=mydomain,dc=net"
directory "/usr/local/openldap/var/openldap-data"
rootdn "cn=admin,dc=mydomain,dc=net"
rootpw mypasswd
index uid,cn,request pres,eq,sub
index mailusage,mailquota pres,eq
index status,mailid,maildrop,lastlogin pres,eq
index objectClass eq
# The admin dn has full write access
access to *
by dn="cn=admin,dc=mydomain,dc=net" write
by anonymous read
by * read
#moduleload back_monitor
database monitor
access to dn.subtree=cn=monitor
by dn.exact=cn=admin,dc=mydomain,dc=net write
by dn.subtree=dc=mydomain,dc=net read
by * none
# Increasing threads from the default
threads 32
# Set the idletimeout for threads
#idletimeout 30
DB_CONFIG:
set_cachesize 2 0 1
set_lg_regionmax 262144
set_lg_bsize 2097152
set_lg_dir /var/log/bdb
# Just use this setting when doing slapadd...
#set_flags DB_TXN_NOSYNC
#set_flags DB_TXN_NOT_DURABLE
micah
----
"Naturally, the common people don't want war, but after all, it
is the leaders of a country who determine the policy...Voice or no
voice, the people can always be brought to the bidding of the leaders.
This is easy. All you have to do is to tell them they are being
attacked, and denounce the pacifists for lack of patriotism and
exposing the country to danger. It works the same in every country."
-- Goering, Nuremburg trial